WASHINGTON Tropical mangrove trees are better at storing climate-warming carbon than most other forests, so cutting them down unleashes far more greenhouse gas than deforestation elsewhere, scientists reported on Sunday.
Mangroves are so efficient at keeping carbon dioxide out of the atmosphere that when they are destroyed, they release as much as 10 percent of all emissions worldwide attributable to deforestation -- even though mangroves account for just 0.7 percent of the tropical forest area, researchers said.
They store two to four times the carbon that tropical rainforests do, said Daniel Donato, of the U.S. Agriculture Department's Forest Service and lead author of a study published in the journal Nature Geoscience.
"Mangroves store a lot of carbon, much more so than most forests on Earth, on a per-hectare basis," Donato said by phone. "Since they store so much carbon, there's probably a lot being released from all the mangrove deforestation that's going on."
Mangroves live where many people want to live, along ocean coastlines in the tropics, and their areal extent -- the amount of land they grow on -- has declined by 30 percent to 50 percent over the last 50 years, the study found.
Coastal development, aquaculture and over-harvesting have all contributed to mangrove deforestation. Rising sea levels expected this century are also a threat, the study said.
Besides storing carbon, mangrove forests act as fisheries, keep sediment in place, produce fiber and protect inhabited areas against storms and tsunamis, the researchers said.
Coastal mangrove forests and the ecological services they provide could be gone in as little as 100 years, according to the researchers.
Part of the reason for mangroves' efficiency in keeping carbon locked away lies in their location in tidal zones, where their roots are often covered with sea water.
They need complex root systems to keep them breathing even when the tides come in, Donato said. This same complexity traps sediment that comes in from rivers and it builds up.
SLOW DECAY MEANS CARBON STORAGE
In most forests, this kind of sediment and litter would decay rather quickly, but because much of it gets submerged in a coastal mangrove forest, there is not enough oxygen to break it down, so the breakdown of materials is much slower.
A slower decay means more carbon dioxide gets stored.
Scientists have previously studied the rate at which mangroves could sequester carbon, but this latest research looked at how much of a pool of stored carbon was locked away in the trees, in their root systems and in the soil decomposing slowly around them.
To figure this out, Donato and his colleagues went to 25 mangrove forests stretching from the Ganges Delta in Bangladesh to Micronesia in the western Pacific, and from the Malay Peninsula in southeast Asia to northern Australia.
Mangroves grow in 118 countries, but the region the scientists chose has the greatest mangrove area and diversity.
They figured out how much total carbon these trees kept out of the air by measuring the trees' size, the tree litter on the forest floor, the amount of carbon in the soil around them and the depth of the soil.
"Mangroves store about two to four times what (tropical rainforests) store, and it's mostly in that thick organic muck layer in the soil," Donato said. "That's really what sets mangroves apart in terms of carbon storage."
(Editing by Eric Walsh)